Thin film devices are often developed and manufactured in large arrays which are created in a batch process. Such arrays of thin film devices generally have rigorous specifications on the size, shape, location, orientation and register in the individual devices and in their ultimate assemblies. The mensurating tasks of point location, codification, data reduction and recording of such inspections can be formidable unless the inspection strategy includes some form of automatic image analysis.
In the case of miniature elements, this instrumentation takes the form of a microscope/closed circuit television module which is pointed and interpreted by a computerized positioning device. Like a military fire control system, this instrumentation must acquire its target and move the system such that the target image is centered to its field of view. Unlike the military fire control system, the nominal location of these targets is designed into their manufacture. Thus, the sophistication and complexity normally required for fire control system image analysis is not needed for these targets.
Many complex and sophisticated systems for pointing, tracking, homing, intercepting and navigation guidance are in existence and use in ordinance, satellite and astronomical disciplines. The cost and sophistication of these systems, however, appear to be an overkill for the problem of positioning a known shape identified within a known field of view. Such classical tracking systems perform many familiar tasks. Including the aforementioned fire control tasks, there are also missile trackers, target trackers, and missile guidance. From navigation and astronomy there are star trackers and horizon sensors. From robotics, there are photomask registerers, tracking theodolites, and many pointing microscopes.
These pointing strategies use a variety of physical phenomena to accomplish their error sensing task in a known media. All sense, interpret, and react. Some use inherent target radiation (passive systems); others are active in that they supply the media. Target shape, range, range rates, angular rate, size, color, spatial, and time frequency are but a few of the parameters that are used successfully to automatically discriminate and lock-on a given target. In optical-mechanical-electronic systems, there must be viewing systems, scanning systems, interpretation processors, error signal generators, and servo systems to accomplish the pointing and, finally, feed-back systems to monitor the success of the tracking. These feed-back systems detect the end point of the operation and "shoot the gun" or initiate some kind of response.
Tracking systems are normally classified by whether they scan the image plane or the object plane; whether they are simple scanners, stationary mosaics, and/or nonreticle or reticle devices. Such various and extensive classification procedures and systems are mentioned only to indicate the massive amount of literature that is available for such problems. For example, one source for additional information is the bibliography of "Advanced Infrared Technology", the text for the University of Michigan Engineering Summer Conferences, June, 1965.
The scanning techniques used in the various tracking systems often depend upon the complexity of the particular system. However, in the field of the present device, a relatively known simple search pattern should be possible because most of the targets are essentially primitive shapes and the optical contrast between and within them and their background is optimum, and in fact is essentially perfect. Well known simple search patterns include the spiral of Archimedes, rosettes, and raster scans. However, such "simple search patterns" still appear to be overly sophisticated for the field of the present invention, particularly where the approximate location and orientation, as well as the shape, of the object to be located is known. In addition, the uniformity and relatively primitive shapes of some product targets, such as those in the advanced ignition system field suggest that the target pattern recognition and lock-on mechanism can be of the most simple design. In short, most conventional target acquisition processors are too sophisticated for the present problem.
There was also found to be a need for a simple optical pointing system in the field of exploding bridge elements which are commonly used to activate flyer plates in laboratory instrumentation that is used for spalling, equation of state, and other investigations involving impact transfers of high energy intensity. The mensuration of such planar objects by profile projection and subsequent automatic image analysis demonstrated a need for a simple optical pointing system.
Previous techniques for inspecting large arrays of precision elements are known. Such techniques include systems for manual positioning and visual inspection by hourly inspectors. The obvious enormity of attempting a nonautomated visual "one-on-one" inspection of these components in vast arrays with manual positioning has also been documented. These documents discuss the required precision and methodology of the techniques but the implicit inspection-time per assembly and operator fatigue demonstrate a need for an automatic image analysis approach. For example, thirteen replications per element are required in one case in order to achieve the required precision for point location. The operator must peer into a microscope to perform optical nulls on the elements and must maintain a rigorous attention span of about 2 to 3 hours per part.
Current practice provides for the nonautomated visual inspection of many such arrays for mensurations and surface flaw detection. Obviously, this practice is also time-intensive, tedious, subjective, and prone to operator-fatigue errors. The need for such visual evaluation by an automatic system is apparent.
With respect to advanced ignition systems, the product consists of precision thin-film patterns deposited on dielectric substrata and may be manufactured in the form of large arrays of individually critical components. This multitude of precision elements must be inspected for location, orientation, size and shape, both as individuals within the array and as groups with respect to their registration with other arrays in the final assembly. The number of individual inspections and the required accuracy of the determinations also demonstate a need for an automatic image analysis as the most prudent approach.
The repeatibility of modern photolithography techniques is such that nominal pointing of an automatic image analysis system will result in positioning a subject element on center with the system's TV image field-of-view to within plus or minus 0.1 mm. However, timing specifications require at least five times that positional accuracy and integrity.
To achieve the required precision of location data, the nominal pointed target must be acquired, its pattern must be recognized, and it must be oriented and moved to the center of the pointing microscope's field-of-view before further mensurations or location algorithms can be employed. The problem may thus be defined as the acquisition of the capability to: point the microscope at the nominal element location; acquire the target image; orient the target or the microscope; and translate the target or the microscope to the center of the pointing system's optical axis. When pointing to the nominal position of the target, the task further becomes one of measuring the target's deviation from its nominal position and moving the target or the optical field of view to the optimum measurable position. Normally with the current accuracy of present day manufacturing processes, the target usually falls into a random location and orientation within the field of view of the optical instrumentation upon a nominal pointing. However, as mentioned above, such accuracy is still not within the accuracy required for the inspection and mensuration of minature elements. It is the need for the further positioning in an automatic method and using automatic apparatus that served as the background for the present invention.
Numerous United States patents discuss the above mentioned overall problems and include the following: Doemens No. 4,253,112; Altman No. 4,233,625; Schmitt et al No. 4,212,031; Schmitt et al No. 4,203,132; Suzki No. 4,167,677; Christy et al No. 4,160,263; Johannsmeier No. 4,070,117; Moriyama et al No. 4,057,347; Dye No. 3,394,366; and Heinz No. 3,207,904.
The Doemens patent is directed to the alignment of a semiconductor wafer and a mask for subsequent processing in which the alignment is accomplished by creating error signals from a difference in the analog sequential signals generated by two contemporaneous single scans. Fiducial patterns are located on both the mask and the wafer.
The Altman patent discloses a system for aligning successive, small, identical, rectangular grid patterns according to a previously selected position and orientation. It is a position duplicator for nearly identical patterns.
The two Schmitt patents are concerned with aligning a body to a preselected coordinate system and the alignment or register between two elements. A closed circuit television with variable magnification is utilized to obtain images of the body. A found feature in the image of the body is digitized and stored as a binary record. The body is then positioned into the desired alignment by servos operating on generated error signals.
The Suzki patent discloses an optical device for aligning semiconductor wafers and masks for successive production operations. The alignment is accomplished by using fiducial marks on both components. An algorithm converts nonsymmetrical signals from a single linear scan of the superimposed images of the fiducial marks into error signals for translation. The algorithm also converts other data from a separate area of the images into error signals for rotation.
The Christy patent discloses a video microscope with multiple objective lenses that can project superimposed images from two or more fields of view.
The Johannsmeier et al patent discloses an apparatus for the alignment of masks and the work in photolithography. A split image for the preselected pattern is sampled through "peek-a-boo" slits and error signals are generated based on the registration of the fiducials.
The Moriyama patent discloses apparatus for minimizing registration errors between a work table and a photomaster table in photographic exposures. The apparatus utilizes a feedback signal generated by the difference between the commanded position of the table and the actual position of the table.
The Dye patent discloses a data display system in which an image is digitized and stored in the memory of a computer, with the image being rearranged via a manually operated light pen.
Finally, the Heinz patent discloses a system for electro-optically positioning articles through translation and orientation. The total light from a quadrant is integrated and then compared with the light from other quadrants with the system attempting to equalize the light per quadrant.